LIDAR systems for measuring wind velocity based on Doppler shift are well known in the literature. Generally, such systems employ coherent detection (heterodyne or homodyne detection) in order to detect the small wavelength shifts associated with the Doppler shift and the weak signals backscattered to a detector of the system, by mixing the backscattered signals with a reference or local oscillator signal, and detecting the mixed signal.
An example of a transmitter for a coherent Doppler LIDAR is disclosed in Marinov, et al., “Using the spectral asymmetry of TEA CO2 laser pulses to determine the Doppler-shift sign in coherent lidars with low frequency stability”, Applied Optics, 38(12), 1999.
To use coherent detection, the coherence length of a laser source used must be at least equal to a round-trip distance from the source to a probe volume and back to the detector system, often on the order of several hundred meters. This is a rather strict requirement, which is typically only met by expensive and sophisticated laser sources.
In coherent detection, the weak signal to be detected is mixed with a relatively strong reference or local oscillator signal, e.g. on the order of 1 mW. While this provides an amplification of the signal to be detected, it also induces a significant shot-noise contribution to the detected signal. This shot-noise is typically the dominant noise source in such systems, and thus limits the obtainable signal-to-noise ratio (SNR)
Another limitation of coherent detection is imposed by phase-induced intensity noise (PIIN), also known as beat or interference noise, which is caused by beating of unwanted internal reflections from the LIDAR system with a local oscillator signal.
In coherent detection, sign of Doppler shift is not determined unless additional means are used (e.g. use of acousto-optic modulators), which increases system complexity and cost.
Hence, an improved LIDAR system would be advantageous, and in particular a LIDAR system with an improved signal-to-noise ratio (SNR) and ability to readily obtain sign of Doppler shift would be advantageous.